This sounds to me like a combination of lifter tick and piston slap, I'm just not sure.. In this video you'll see it just about the worst I've heard it so far. I've had the car for about 500 miles.
It's a 2006 IX with almost 71k miles, completely bone stock.
Also, could it be a worn out tensioner? I don't have any tools where I'm at right now.
http://www.youtube.com/watch?v=H-mDc...ature=youtu.be

 

Sounds like lifters gone bad. I recently changed mine to gsc no tick lifters.. but it's hard to tell from the video. Rev it very slowly to 3k rpm and see if you here ticking noise. If you hear ticking from idle to 3k rpm, it's most likely lifters.

 

Also, just like with lifter tick it's not constant or consistent.